netdev - Re: issue with inflight pages from page

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Message-ID: <ZD5IcgN5s9lCqIgl@lore-desk>
Date:   Tue, 18 Apr 2023 09:36:18 +0200
From:   Lorenzo Bianconi <lorenzo@...nel.org>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Eric Dumazet <edumazet@...gle.com>, netdev@...r.kernel.org,
        hawk@...nel.org, ilias.apalodimas@...aro.org, davem@...emloft.net,
        pabeni@...hat.com, bpf@...r.kernel.org,
        lorenzo.bianconi@...hat.com, nbd@....name
Subject: Re: issue with inflight pages from page_pool

> On Mon, 17 Apr 2023 23:31:01 +0200 Lorenzo Bianconi wrote:
> > > If it's that then I'm with Eric. There are many ways to keep the pages
> > > in use, no point working around one of them and not the rest :(  
> > 
> > I was not clear here, my fault. What I mean is I can see the returned
> > pages counter increasing from time to time, but during most of tests,
> > even after 2h the tcp traffic has stopped, page_pool_release_retry()
> > still complains not all the pages are returned to the pool and so the
> > pool has not been deallocated yet.
> > The chunk of code in my first email is just to demonstrate the issue
> > and I am completely fine to get a better solution :) 
> 
> Your problem is perhaps made worse by threaded NAPI, you have
> defer-free skbs sprayed across all cores and no NAPI there to 
> flush them :(

yes, exactly :)

> 
> > I guess we just need a way to free the pool in a reasonable amount 
> > of time. Agree?
> 
> Whether we need to guarantee the release is the real question.

yes, this is the main goal of my email. The defer-free skbs behaviour seems in
contrast with the page_pool pending pages monitor mechanism or at least they
do not work well together.

@Jesper, Ilias: any input on it?

> Maybe it's more of a false-positive warning.
> 
> Flushing the defer list is probably fine as a hack, but it's not
> a full fix as Eric explained. False positive can still happen.

agree, it was just a way to give an idea of the issue, not a proper solution.

Regards,
Lorenzo

> 
> I'm ambivalent. My only real request wold be to make the flushing 
> a helper in net/core/dev.c rather than open coded in page_pool.c.
> 
> Somewhat related - Eric, do we need to handle defer_list in dev_cpu_dead()?

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